1. Technical Field
The present invention relates to a method of manufacturing a nanocomposite using expanded graphite, and, more particularly, to a method of manufacturing a nanocomposite by dispersing expanded graphite in a polymer, wherein the expanded graphite is obtained by separating regularly and strongly bonded plate structures from graphite.
2. Description of the Related Art
Graphite is a mineral having a hexagonal plate-shaped crystal structure and belonging to a hexagonal crystal system. Graphite is black, has a metallic luster, is electrically conductive, and is used for pencil leads and for electrodes for crucibles, electric furnaces, arc furnaces, and the like, and is used as a solid lubricant. Graphite is most characterized in that it has high heat resistance, impact resistance, and corrosion resistance, has a very small thermal expansion coefficient, and has excellent thermal conductivity and electrical conductivity compared to other materials. Further, graphite is also characterized in that its various physical properties such as shape, color, gloss, hardness, specific gravity, thermal conductivity, electrical conductivity, etc., are changed depending on its crystal structure and microstructure.
Natural graphite is a mineral consisting of carbon, and has a hexagonal plate-shaped crystal structure and belongs to a hexagonal crystal system. Graphite is a scale-like multilayered material, and each layer is provided therein with strong carbon covalent bonds (sp2 hybrid), but layers are connected to each other by Van der Waals' force applied therebetween. For this reason, graphite has a plate-like structure having a perfect cleavage plane, and has half-metallic properties of both a metal and a semiconductor.
In graphite, the height of one hexagonal plate-shaped layer is 3.40 Å, and the distance between most adjacent carbons in a hexagonal ring is 1.42 Å. Further, the distance between the upper plate-shaped layer and lower plate-shaped layer is much greater than the distance between the centers of two carbon atoms. For this reason, electrons located over or under the hexagonal plate-shaped layer can freely move, and thus graphite has excellent electrical conductivity. Since diamond, which is allotropic to graphite, has four electrons bonded with each other by strong covalent bonds (sp3 hybrid), it becomes an excellent electric insulator.
When crystalline flaky graphite is oxidized with concentrated sulfuric acid and a hydrogen peroxide solution, washed with water, and then put into a high-temperature expansion furnace, it is expanded in the direction of a c axis of graphite crystal. The crystalline flaky graphite expanded to 100˜700% of initial volume thereof is referred to as “expanded graphite”. This expanded graphite is used in materials for heat-resistant equipment, materials for steel casting, materials for ingot covers, electrodes for steel-making furnaces, high-elasticity refractory materials for furniture and mattresses, and the like. Recently, expanded graphite has been actively used in heat radiation materials for electrical appliances, thermal conduction sheets, fire retardants, conductive fillers, semiconductor parts, materials for light emitting displays (LEDs), materials for field emission displays, and the like. When carbon nanotubes (CNTs), similar to graphite, are synthesized and then refined, the yield thereof is very low. Therefore, even when synthesis is conducted using inexpensive materials, the final product of carbon nanotubes is expensive, whereas the final product of graphite is very inexpensive. Single-wall carbon nanotubes (SWNTs) are characterized in that the metallic properties and semiconductor properties thereof are changed depending on the chirality and diameter thereof, and in that the band gaps thereof are different from each other although they have the same semiconductor properties. Therefore, in order to use specific semiconductor properties and metallic properties from the single-wall carbon nanotubes (SWNTs), it is required to separate all of the single-wall carbon nanotubes (SWNTs). However, it is known that it is very difficult to completely separate these single-wall carbon nanotubes (SWNTs).
In the 1960's, polymer-based composites appeared as a new paradigm in the field of materials. Currently, in addition to the polymer-based composites, polymer nanocomposites are being actively developed. Polymer nanocomposites may be manufactured to have high strength, excellent durability, and various functions. Such a polymer nanocomposite is disclosed in the publication Nature Nanotechnology, entitled “Functionalized Graphene Sheets for Polymer Nanocomposites”, jointly researched by study groups in the U.S.A and U.A.E. Further, the Georgia Institute of Technology in the U.S.A and CNRS in France reported that a transistor and a loop circuit were manufactured using graphene, which is a thin graphite layer. Further, according to the journal “Nano Letters”, which is a journal of the American Chemical Society, Kostya Novoselov and others reported that graphene was used as a transparent conductive coating agent for an electro-optical device. In addition, researchers of the Max Planck Institute in Germany reported that a transparent electrode for a solar battery was developed using a graphene-based film. KR-A-2009-0065206, KR-A-2009-0017454, and KR-A-2009-0028007 also disclose methods of manufacturing an electronic device, such as a sheet, a solar cell, or the like using graphene, and KR-A-2009-0086536 discloses a method of manufacturing a functional graphene-rubber nanocomposite.